For the first three million years of human history, we lived according to circumstance. Our lives were ruled by the happenstances of ecology. We existed, as all animals do, in fear of hunger, predation, weather and disease.

For the following few thousand years, after we had grasped the rudiments of agriculture and crop storage, we enjoyed greater food security, and soon destroyed most of our non-human predators. But our lives were ruled by the sword, the axe and the spear. The primary struggle was for land. We needed it not just to grow our crops but also to provide our sources of energy - grazing for our horses and bullocks, wood for our fires.

Then we discovered fossil fuels, and everything changed. No longer were we constrained by the need to live on ambient energy; we could support ourselves by means of the sunlight stored over the preceding 350 million years. The new sources of energy permitted the economy to grow - to grow sufficiently to absorb some of the people expelled by the previous era's land disputes. Fossil fuels allowed both industry and cities to expand, which permitted the workers to organise and to force the despots to loosen their grip on power.

Fossil fuels helped us fight wars of a horror never contemplated before, but they also reduced the need for war. For the first time in human history, indeed for the first time in biological history, there was a surplus of available energy. We could keep body and soul together without having to fight someone else for the energy we needed. Agricultural productivity rose 10 or 20 fold. Economic productivity rose 100 fold. Most of us could live as no one had ever lived before.

And everything you see around you results from that. We have been able to assemble here from all corners of the country because of fossil fuels. We have not been charged and cut down by the yeomanry - or not yet at any rate - because of fossil fuels. Our freedoms, our comforts, our prosperity are all the result of fossil fuels.

Ours are the most fortunate generations that have ever lived. Ours are the most fortunate generations that ever will. We inhabit the brief historical interlude between ecological constraint and ecological catastrophe.

I don't have to remind you of the two forces which are converging on our lives. We are faced with an impending shortage of the source of energy which is hardest to replace - liquid fossil fuels. And we are faced with the environmental consequences of the fossil fuel burning which has permitted us to be standing here now. The structure, the complexity, the diversity of our lives, everything we know, everything that we have taken for granted, that looked solid and non-negotiable, suddenly looks contingent. All this is a great tottering pile balanced on a ball, a ball that is about to start rolling downhill.

I hear people talking about the carbon cuts they would like to see. I am not interested in what people would like to see. I am interesed in what the science says. And the science is clear. We need not a 20% cut by 2020; not a 60% cut by 2050, but a 90% cut by 2030. Only then do we stand a good chance of keeping carbon concentrations in the atmosphere below 430 parts per million, which means that only then do we stand a good chance of preventing some of the threatened positive feedbacks. If we let it get beyond that point there is nothing we can do. The biosphere takes over as the primary source of carbon. It is out of our hands.

The notion that we can achieve this by replacing fossil fuels with ambient energy is a fantasy. It is true that we have untapped sources of energy in wind, waves, tides and sunlight, but it is neither so concentrated nor so consistent that we can plug it in and carry on as before.

A cut like this requires massive reductions in our energy use. There are some technofixes available, but they are unlikely to take us more than halfway there. If carbon emissions are to be capped at 10%, energy use will have to be capped at under 50%. The only fair means of doing this is national rationing accompanied by global contraction and convergence.

And we find ourselves in an extraordinary position. This is the first mass political movement to demand less, not more. The first to take to the streets in pursuit of austerity. The first to demand that our luxuries, even our comforts, are curtailed.

These are the greatest political challenges any movement has faced. But we are rising to it. We are rising. But let no one tell you it will be easy. If it were just a matter of slagging off George Bush, we would have won by now. But we must struggle not only against him, not only against our own government, not only against each other, but also against ourselves. The struggle against climate change is a struggle against much of what we have become. It is a struggle against some of our most fundamental urges.

We cannot call on others to stop flying if we still fly. We cannot ask the government to force us to change if we are not ready to change. The greatest fight of our lives will be fought not just out there, but also in here.

Editor's NotesThe submitter of the essay writes: "The big picture - Monbiot at his best."
-BA